1. Field of the Invention
The present invention relates to a numerical controller for controlling a machine tool carrying out thread cutting, and more particularly to a numerical controller having a function of re-machining a thread cutting cycle.
2. Description of the Related Art
In the following description, reattaching a thread which is once detached from a chuck of a machine tool to the chuck so as to machine the thread, in the case where a thread which is interrupted to be machined due to breakage of a tool is re-machined, or in the case where a finish machining of the thread is achieved by the other machine after the thread is roughly machined or intermediately machined, is referred to as “re-machining of thread”.
FIGS. 1A and 1B are views illustrating a thread which is reattached to a chuck fixed to a spindle of a machine tool.
A thread is attached to a chuck claw 11 of a chuck 10 which is fixed to a spindle of a machine tool (not shown). In the case where the thread is once detached from the chuck 10 and the thread is thereafter reattached to the chuck 10, a thread cutting start position of the reattached thread 12 is deviated at a phase θ of a screw thread from a threading cutting start position of the thread before the reattachment.
A technique relating to the thread re-machining is disclosed in Japanese Patent Application Laid-open No. 64-9127 and Japanese Patent Application Laid-open No. 60-228020.
The previously cut thread groove is re-machined by changing a timing for starting movement of a feed axis while taking into consideration a delay of a servo system, a delay from detection of pulse data of a spindle rotating speed to an end of computation, a delay caused by acceleration and deceleration, and an amount θ of phase deviation of the screw thread caused by the reattachment of a workpiece (a thread in the process of machining) to the chuck. In the following description, a workpiece is referred to as a thread in the process of machining.
The amount θ of phase deviation of the screw thread caused by the reattachment of the workpiece to the chuck can be calculated by measuring a position of the thread groove of the workpiece which is attached to the chuck for re-machining. In other words, the amount θ of phase deviation of the screw thread can be calculated from a position in a feed axis direction which is measured by reattaching the thread in the process of machining to the chuck 10, and thereafter applying a cutting tool to the thread groove of the thread (the reattached thread 12).
FIG. 2 is a view describing a method of measuring the position of the thread groove.
The thread is reattached to the chuck 10 which is fixed to the spindle of the machine tool (not shown), and the chuck 10 is stopped at a reference position. Then, a cutting edge 15a of a cutting tool 15 is applied to a groove of the thread 12 which is reattached to the chuck 10. Further, a numerical controller controlling the machine tool stores a position ZME (mm) of the thread groove in a feed axis direction for starting the thread cutting and a position ZST (mm) in the feed axis direction in a memory device in the numerical controller, at a time of inputting a signal or issuing a command according to a machining program.
Accordingly, the amount θ (rev) of the phase deviation of the screw thread due to the reattachment can be calculated by the following expression (1).
                              θ          =                                                                                                          Z                    ME                                    -                                      Z                    ST                                                                              -                              L                ×                e                                      L                          ⁢                                  ⁢                  e          =                      [                                                                                                Z                    ME                                    -                                      Z                    ST                                                                              /              L                        ]                                              (        1        )            
In the above expression, L (mm) represents a lead (pitch) of the thread, and mark [x] is a Gaussian symbol indicating a maximum integer which does not go beyond x.
In the thread cutting cycle, the thread cutting based on a one-edge cutting or a zigzag cutting is frequently used. The one-edge cutting or the zigzag cutting is a machining method having an advantage that a cutting resistance can be reduced and a disposal capacity of chips can be improved, by using a piece of one side of a cutting edge of a cutting tool for cutting.
FIG. 3 is a view illustrating the one-edge cutting. In the one-edge cutting, a workpiece is machined by using a piece of a cutting edge 15a (a left piece of the cutting edge 15a in an example in FIG. 3) of the cutting tool 15. FIG. 4 is a view illustrating the zigzag cutting. In the zigzag cutting, the workpiece is machined by using each of pieces of the cutting edge 15a (right and left pieces of the cutting edge 15a in an example in FIG. 4) of the cutting tool 15 one after the other.
However, in the conventional re-machining according to the thread cutting cycle, a radial incision forming a vertical incision into a thread grove, as shown in FIG. 5, is carried out in place of the one-edge cutting or the zigzag cutting. As a result, there is a problem that not only the advantage of the thread cutting based on the one-edge cutting cannot be obtained, but also a phase is deviated from an originally expected screw thread phase. In FIG. 5, reference numeral 17 denotes a measuring position of the thread groove, reference numeral 18 denotes a shape of the screw thread using the one-edge cutting which is originally expected, and reference numeral 19 denotes a deviation of the screw thread which is generated by the re-machining on the basis of the radial incision.
In the thread cutting cycle, for the purpose of the reduction of the cutting resistance and the improvement of the chip disposal capacity, the thread cutting is carried out using the one-edge cutting (refer to FIG. 3), or the thread cutting is carried out using the zigzag cutting (refer to FIG. 4). In the thread cutting using the one-edge cutting, the thread cutting is carried out by deviating the position for starting the thread cutting in the feed axis direction according to the number of incisions, as shown in FIG. 3. Even in the thread cutting using the zigzag cutting, it is necessary to reflect an amount of deviation of the phase in the screw thread caused by the reattachment of the workpiece (the thread in the process of machining) on the timing for starting the movement of the feed axis, in the same manner as the thread cutting using the one-edge cutting.
It is found that, in order to determine the amount θ of deviation of the phase of the screw thread, it is necessary to know the position ZST in the feed axis direction for starting the thread cutting, on the basis of the above expression (1). However, since the position ZST in the feed axis direction for starting the thread cutting varies on the basis of the number of incisions, the position ZST in the feed axis direction for starting the thread cutting cannot be obtained until how many incisions have been made until the measured thread groove is machined is known. As a result, it is impossible to determine the amount θ of phase deviation for carrying out the one-edge cutting.
The conventional method of measuring the thread groove does not have any means for specifying the number of incisions, and cannot calculate the amount of phase deviation for carrying out the thread cutting using the one-edge cutting or the zigzag cutting. As a result, in the thread re-machining according to the thread cutting cycle, a radial incision in which incisions are carried out in a manner such that the position in the feed axis direction, stored by the measurement, forms the thread groove, as shown in FIG. 5. Therefore, not only the advantage of the thread cutting using the one-edge cutting or the zigzag cutting cannot be obtained, but also there is a problem that the phase is deviated from the originally expected phase of the screw thread.